The use of x-rays for radiographic imaging is well known. Radiographic imaging, at least in crude form, has been known since approximately the time of discovery of x-rays. In its simplest form, imaging is conducted by providing a source of x-ray radiation, an object to be imaged through which the x-ray radiation is transmitted, and a detector which serves to detect and record the transmitted radiation. The most common and simplest form of detector is x-ray sensitive photographic film. In a conventional set-up, radiation is transmitted through the object to be imaged and then is detected by a substantially two dimensional plane of x-ray sensitive film. Radiation is incident upon the film in a substantially perpendicular direction to the plane of film. Typically, such x-ray sensitive film is thin compared to the x-ray stopping distance of the film. Such film detection, as well as most of the x-ray receptors currently available for radiographic imaging, offer poor performance with respect to x-ray stopping power, signal collection efficiency, and read out efficiency, that is the number of read out elements needed to achieve suitable detection. For example, x-ray fluorescent phosphorus screens which are used with film are limited in thickness in order to avoid excessive optical selfattenuation and loss of spatial resolution.
Though this self-attenuation problem may be overcome, see for example, U.S. Pat. No. 4,560,882 to R. Nelson, Z. Barbaric entitled High Efficiency X-Radiation Converters, problems still remain. Several energy conversion stages are required such as x-ray-to-optical-to electronic signal, which may prove inefficient. If film is the optical sensor, then the film must be optically scanned if a digital image is desired.
X-ray detectors which directly provide an electrical output signal, thereby eliminating the need to optically scan an intermediate image such as on a film or sense the fluorescent signal with a photodetector, have proved difficult to implement in practice. Solid state x-ray detectors have been constructed from materials such as amorphous selenium, U. Schiebel, et al., Proceedings of the Society of Photo-Optical Instrumentation Engineers, 626:176, 1986, CdTe,Ge,HgI.sub.2, PbO, GaAs, and Si, Y. Naruse, et al., IEEE Transactions in Nuclear Science, Volume NS28, No. 1:47, 1981, D. Miller, et al., IEEE Transactions in Nuclear Science, Volume NS-26, No. 1:603, 1979. Such materials have proved to be difficult to manufacture into plates or linear or two dimensional arrays with adequate thickness. Problems arise due to manufacturing imperfections. Additionally, possibly very high voltages are needed for a large thickness of material.
Improvements have been made upon the basic fluorescent phosphorus screen detectors. One such improved detector is a laser scanned storage phosphor detector, as shown for example in M. Sonoda, et al., Radiology, Volume 148:833, 1983. Such detectors operate by placing the phosphor in a metastable state upon detection of an x-ray, which is subsequently detected by scanning the phosphor with a laser beam which causes excitation of the phosphor from the metastable state to a higher energy state followed by subsequent de-excitation to a ground state. While such laser scanned storage phosphor systems provide a direct electronic read out, they suffer the same limitation experienced by x-ray phosphor screens, that being optical selfattenuation with increasing thickness.
Yet further problems arise in conjunction with electronic read out systems. While it is desirable to provide a direct electronic read out from a detector, it may be difficult to make a large two dimensional detector with high resolution and high stopping power for x-rays since many read out elements may be required. The use of many read out elements translates into a large number of interconnections, which result in the typical manufacturing and reliability problems experienced in such systems.